I tend to agree with the others here, and would not want anything much under 30 ft for 30 days with 2 adults and 2 young kids. Even at 30 ft you will likely find yourself on top of each other most of the time. I have a 28 ft class A that I bought in 2016, my longest trip to date has been about 30 days and 4200 miles, which I feel was really pushing it. This was a loop from Louisiana to Wyoming, with about 2 weeks spent in Wyoming, I drove up and back on my own, and my wife flew up and met me for the Wyoming portion of the trip (she had work obligations that limited when she could take vacation days) , 4 days driving up (1350 miles) through Texas, Oklahoma and Colorado, 9 days on the more leisurely return drive through Nebraska, Kansas, Missouri and Arkansas (1380 miles though it felt longer).
As to your concern about vehicle size, on the open road size itself will not be an issue, though handling may be, just remember, slow down you are on vacation. You will be amazed how much better an RV handles at 55 mph vs 65 mph, and forget about trying to keep the speed limit which is 75 or 80 mph in much of the region. Also you will have plenty of open road once you leave Las Vegas in just about every direction to get comfortable driving the coach, the first 100 miles are so may be a bit stressful, but you will get the hang of it, the next 200 will be better, and by the time you have drive 500 or so miles it will almost be second nature. Just remember you are a lot taller than you are in your regular car, also a bit wider and longer, but taller is the one that tends to get people in trouble, watch for overhanging limbs (though this will not be much of an issue around Vegas as there are few trees).
